Náttúrufræðingurinn - 2012, Síða 144

Náttúrufræðingurinn
144
Þakkir
Rannsóknamiðstöð Íslands (áður Rannsóknarráð Íslands) hefur styrkt þátt-
töku Íslands í djúpborunarverkefnum á Grænlandi. Einnig hafa rannsóknar-
sjóður Rannís og Tækjakaupasjóður styrkt vísindarannsóknir á djúpkjörn-
unum með margvíslegum hætti. Rannsóknarsjóður Háskólans hefur
einnig styrkt rannsóknirnar.
Heimildir
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vegasaltið milli suður- og norður-
hvels (e. north-south thermal see-
saw) og er skýrður með breytingum
á hringrás heimshafanna. Stöðvist
djúpsjávarmyndun, sem á sér stað
á hlýskeiðum norður af Íslandi,
berst hingað norðureftir minna af
hlýjum hafstraumum úr suðri. Þetta
veldur snöggkólnun á norðurhveli
en hægfara hlýnun og síðan kólnun
í suðurhöfum og í kringum Suður-
skautslandið.
Summary
Paleotemperature reconstruction
from Greenland deep ice-cores
Research on Greenland deep ice-cores
shows that the ice sheet archives span
at least 123,000 years of continuous
paleoclimate data. The deep ice cores
demonstrate that climate has been
highly unstable during the last glacia-
tion when temperature oscillated 25
times between 8–15 °C, with the dura-
tion of the warmer episodes between
1000 to 2000 years. The warming oc-
curred in a matter of decades, followed
by a more gradual cooling. At the end
of the last glaciation some 11,700 years
ago climate changed dramatically in 3
to 50 years, depending on the variable
studied. The most rapid changes are ob-
served in the deuterium excess, that re-
flects changes in the moisture source
area, whereas changes in averaged an-
nual temperature on the Greenland ice
sheet varied about 8°C over 40 years.
The changes from the last interglacial
period, the Eemian, into the last glacial
period that started some 122,000 years
ago were according to the isotopic
record of the NGRIP ice core much
slower or about 7000 years. The last
interglacial period seems to have been
about 5°C warmer than at present.
However, this seems not to have greatly
affected the thickness of the ice sheet at
the drilling sites except for the Dye-3
site located in southeast Greenland
where the glacier was about 500 m
lower at that time due to ice melting.
Compared to the last glaciation the cur-
rent Holocene epoch has been stable.
When studied in detail it becomes how-
ever evident that climatic changes have
occurred during this 11,700 years pe-
riod. The most rapid and severe change
occurred 8,200 years ago, when temper-
ature dropped by about 4°C for 100 to
200 years. Cold periods also occurred
11,300 years and 9,300 years ago. The
warmest period during Holocene, the
climatic optimum, occurred ca 9000 to
6000 years ago, followed by a gradual
decline until about two millennia ago.
Cold periods are evident in the ice cores
at the end of the 14th and 17th centuries
and the warming at the beginning of
the 20th century is also very distinct.
Comparison between the Greenland
and Antarctic ice cores suggest that
the Dansgaard-Oeschger events in
Greenland are related to the Antarctic
Isotope Maxima by means of the Bi-
polar seesaw, triggered by North
Atlantic Ocean circulation.